1. Field of the Invention
The present invention relates to a method for fabricating a solid-state image sensor, and, more particularly, to a method for fabricating a solid-state image sensor using self-alignment to form an optical shielding metal (OSM) layer, thus improving its sensitivity and solving the smear problem.
2. Discussion of the Related Art
A solid-state image sensor is a device which images a subject and then outputs the imaged subject as electrical signals by using photoelectric conversion devices and charge-coupled devices (CCDs). By using potential changes in the substrate, CCDs transfer in a predetermined direction charges generated in the photoelectric conversion devices, e.g., photodiodes.
A general solid-state image sensor includes a plurality of photoelectric conversion regions, e.g. photodiodes, vertical charge coupled devices (VCCDs) formed between the photoelectric conversion regions for transferring in a vertical direction the charges generated in the photoelectric conversion regions, a horizontal charge coupled device (HCCD) for transferring in a horizontal direction the charges transferred in a vertical direction by the VCCDs, and a floating diffusion region for sensing and amplifying the charges transferred in a horizontal direction and then outputting the charges to a peripheral circuit.
A conventional solid-state image sensor will now be described with reference to the accompanying drawings.
FIG. 1 is a cross-sectional view showing a structure of a solid-state image sensor, which includes a p-type well 2 formed beneath the surface of an n-type semiconductor substrate 1. Buried charge coupled devices (BCCDs) 3 are formed in the p-type well 2 and spaced apart from one another, PD-N regions 4 are formed in the p-type well 3. Channel stop layers 5 are formed partially around the PD-N regions 4. PD-P regions 6 are formed on the PD-N regions 4. A gate insulating layer 7 is formed on the entire surface including the PD-N and PD-P regions 4 and 6 and the BCCDs 3. Polygates 8 are formed over the BCCDs 3 and above the gate insulating layer 7. An insulating layer 9 is formed to cover the polygates 8. OSM layers 10 are formed on the entire surface except over the PD-N regions 4. At this time, the BCCDs 3 stand for VCCDs.
An operation of a conventional solid-state image sensor having such a structure will now be described. Charges accumulated in photoelectric conversion regions, e.g., the PD-N regions 4 and the PD-P regions 6, which convert the image into electric signals, are transferred to BCCDs 3 by clock signals, applied to a transfer gate (not shown). Then, the charges transferred to the BCCDs 3 are transferred in a vertical direction by clock signals applied to the polygates 8. The OSM layer 10, formed on the gate insulating layer 7 and the insulating layer 9 (except over the PD-N regions 4), prevents photons from directly entering the BCCDs 3.
FIG. 2 is a cross-sectional view showing problems of the conventional solid-state image sensor. In general, the OSM layer 10 is made of a metal having a 99% reflectivity.
A process for forming an OSM layer in a conventional solid-state image sensor will now be described. First, a gate insulating layer 7 is formed on the entire surface including the photoelectric conversion regions, i.e. the PD-N and PD-P regions 4 and 6 and the BCCDs 3. Subsequently, the polygates 8 are formed on the gate insulating layer 7 and only on the BCCDs 3, and then the insulating layer 9 is formed on the entire surface of the polygates 8. Thereafter, an OSM layer 10 is formed on the insulating layer 9 and the gate insulating layer 7, except over the PD-N regions 4. To form the OSM layer 10, optical shielding metal is formed on the entire surface of the insulating layer 9 and is selectively patterned by photolithography and photo etching processes. The insulating layer 9 is selectively removed over the PD-N regions 4.
However, as the number of pixels increases in order to obtain a miniaturized device with good resolution, minimum line width is reduced, and margins for the photolithography is also reduced. Accordingly, if misalignment exists, as shown in FIG. 2, the OSM layer 10 can shift over a portion of the PD-N region 4. If so, a smear can result, where light is smeared into the CCDs (mainly into VCCDs) at one side of a light-receiving part (PD-N) where the OSM layer 10 is not formed. At this time, the smear, where carriers due to light leakage are directly transferred to adjacent CCDs, can cause a phenomenon where a trail is dragged in a vertical direction on a screen.
FIG. 3 is a cross-sectional view showing a structure of another conventional solid-state image sensor. The structure of this solid-state image sensor is similar to that of the solid-state image sensor shown in FIG. 1. The difference between these two sensors, however, is that an OSM layer 10 of FIG. 3 covers the edge portions of the PD-N region 4. The solid-state image sensor shown in FIG. 3 is designed to solve the problem generated in a solid-state image sensor shown in FIG. 2. Since the OSM layer 10 is expanded by a predetermined distance D, light is blocked from transmission through PD-N regions 4 in spite of misalignment.
However, such conventional solid-state image sensors have the following problems. First, as the number of pixels increases to obtain miniaturized devices with light weight and high resolution, process margins for photolithography decrease. Accordingly, the possibility of misalignment and smear is high, thus affecting the reliability of a solid-state image sensor.
Second, in the conventional solid-state image sensor as shown in FIG. 3, an OSM layer is formed over edges of a PD-N region in order to prevent the smear. However, total light-receiving area is decreased even though the OSM layer is precisely patterned, thereby degrading its sensitivity. If misalignment also exists, the sensitivity of the device becomes inferior, and its performance deteriorates.